Endoscopic, morphological, histochemical and biophysical properties of the duodenal mucosa as well as secretory activity of the stomach and pancreas were studied and compared in 398 patients with duodenal ulcer in different phases (active ulcer, healing ulcer, red and white scars). Sixteen patients were observed over time. Analogous studies were conducted in 88 patients with chronic diseases of the biliary and duodenopancreatic zone (with non-ulcerous duodenitis) and in 32 normal persons. The duodenal mucosa was established to undergo consistent morphofunctional reorganization related to the disease phase. Active duodenal ulcer was marked by duodenitis with thickening of the mucosa, shortening of the villi, reduction of the depth of the cryptae, gastric metaplasia, impairment of histochemical properties of epitheliocytes and decrease of the transmural potential difference, overproduction of acid and pepsin in the basal period, high sensitivity of the chief cells to stimulation, and insufficient output of bicarbonates by the pancreas. Gastric metaplasia of the duodenal epithelium, appreciable histochemical and morphometric alterations may be regarded as relatively specific traits of peptic duodenitis. As ulcer heals, a well-defined tendency may be noted towards normalization of all the parameters mentioned. However, they only correspond to normal in 25% of patients in the phase of a white scar. The conclusion is made that there is also a necessity of correcting morphofunctional abnormalities after duodenal ulcer healing.